I am on O2 (iPhone) and have an unreliable signal round the Watton at Stone By Pass but it is okay through the villages. My OH commutes to Stevenage and he is on Orange (Samsung Galaxy old model) and has the same problem but I would say his is marginally better.

We are on Three and have had major issues with getting a signal when our car broke down. Although generally we're very happy with Three we want a cheap PAYG emergency phone for when we can't get a Thee signal.